1. Field of the Invention
This invention generally relates to processing signals, and more particularly to a system and method for filtering signals in a transceiver of a communications system.
2. Description of the Related Art
A transceiver is an integrated device which transmits and receives signals in a communications system. Transceivers generally operate in one of two modes. If the transceiver is unable to receive signals while transmitting, it is said to operate in half-duplex mode. If the transceiver is able to receive signals while transmitting, it is said to operate in full-duplex mode. Full-duplex mode transceivers make up the predominant share of all RF transceivers in use today, however half-duplex mode transceivers still perform many important applications.
The receiver part of a transceiver includes a front-end portion and a signal-processing portion. The front-end portion performs the function of baseband signal recovery and is important from a designer's standpoint because its noise figure and linearity determine the overall performance of the transceiver. The signal-processing portion processes the baseband signal according one of a variety of wireless standards.
The manner in which baseband signal recovery is performed may be used as a basis for classifying the receiver. If baseband recovery is performed in two steps (using two local oscillator signals), the receiver may be classified as a super-heterodyne receiver. If baseband recovery is performed in one step (using one local oscillator signal), the receiver may be classified as a homodyne receiver.
FIG. 1 shows the receiver portion of a super-heterodyne transceiver which recovers a baseband signal in two down-conversion stages. In a first down-conversion stage, a received signal is mixed with a first oscillator signal (local oscillator LO1) in mixer 1 to generate an intermediate frequency signal. In a second down-conversion stage, the intermediate frequency signal is mixed with a second oscillator signal (LO2) in mixer 2 to generate a baseband signal, which is subsequently filtered for input into the signal-processing portion of the receiver. The oscillator signals are generated by respective phase-locked loop circuits 3 and 4, and a number of filters 5–8 and amplifiers 9 and 10 may be included to process the signal at various stages along the baseband signal recovery path. A transmitter portion of the transceiver (not shown) uses two up-conversion stages to convert a baseband signal to a predetermined carrier frequency.
FIG. 2 shows the receiver portion of a homodyne (direct-conversion) transceiver which recovers a baseband signal in one down-conversion stage. The receiver portion includes a low-noise amplifier 11, a down-converter unit 12, and two baseband circuits 13 and 14 for processing respective I and Q signals. The down-converter unit includes mixers 15 and 16 which mix the I and Q signals from the low-noise amplifier with phase-shifted versions of a local oscillator (LO) signal to thereby recover the baseband signal. Unlike a super-heterodyne receiver, the local oscillator signal is set to the frequency of the received signal (i.e., the carrier frequency). This causes the baseband signal to be recovered without the use of an intermediate frequency, hence the name “direct-conversion” receiver. The baseband circuits process the filtered I and Q signals using a number of low-pass filters 17 and 18 and variable-gain amplifiers 19 and 20. The signals output from the baseband circuits are then converted into digital signals by A/D converters 21 an 22 and input into a signal-processing portion of the receiver. A transmitter portion of the transceiver uses a single up-conversion stage to convert a baseband signal to a predetermined carrier frequency.
Super-heterodyne transceivers have drawbacks when used in multi-mode and other applications. For example, the filters located in advance of the IF mixer are typically band-pass filters having a high quality factor. It has been shown to be very difficult to achieve high accuracy and low loss using filters of this type in an integrated form. Accordingly, super-heterodyne transceivers often use passive filters along their baseband signal recovery paths. Unfortunately, these passive filters have limited flexibility for supporting many wireless standards.
The homodyne (direct-conversion) receiver is emerging as the receiver of choice among mobile system designers because it is able to perform multi-mode/multi-band applications. This type of receiver is also preferable because it can support a variety of wireless standards (e.g., 802.11b, GSM, and Bluetooth) using less hardware than a heterodyne receiver. Further, direct-conversion receivers are able to replace many of the passive filters in the heterodyne architecture with integrated low-pass filters, which, for example, may correspond to any one of a number of high-performance analog filters.
The substitution of integrated low-pass filters for passive filters may be preferable for a number of reasons. For example, while SAW and other types of passive filters demonstrate improved performance compared with integrated filters, passive filters have fixed characteristics and therefore critical parameters such as pass-band width and center frequency cannot be changed. To overcome this shortcoming, multiple passive filters must be used to support an equal number of operational bands and communication standards.
On the other hand, all the electrical characteristics of an integrated filter can be controlled and thus multi-functionality can be provided. Accordingly, when used in a multi-mode system, a single integrated filter can support multiple standards simply by changing the characteristics of the filter based on the incoming signal. Also, while performance of an integrated filter tends to be lower than a passive filter in terms of linearity and sensitivity, these performance drawbacks can be overcome by combining the integrated filter with other circuit building blocks such as a gain-controlled amplifier.
FIG. 3 shows an active RC implementation of an integrated analog low-pass filter typically used for channel selection purposes in the baseband signal recovery portion of a direct-conversion receiver. This specific implementation is a 3rd-order elliptic filter having three differential amplifiers 25–27 connected in series. The characteristics of the filter (e.g., cut-off frequency) are determined based on values selected for the variable capacitors C1–C5 and resistors R1–R5. In this implementation, the capacitors have capacitances which may be varied to change the characteristics of the filter.
While there are advantages to using integrated analog low-pass filters in performing baseband signal recovery compared to passive filters, the former type of filter has at least two drawbacks as used in conventional transceivers. First, this type of filter consumes an excessive amount of chip area, which can mostly be attributed to the size and space requirements of their capacitors and resistors. This undermines the ability to miniaturize the receiver portion of the circuit and consequently the transceiver chip in general.
Second, conventional transceivers use separate baseband filters along their transmitter and receiver paths. The receiver filter is used for channel selection and the transmitter filter is used for suppressing spurious signals from digital processing blocks such as digital-to-analog converters. Because these filters perform very different functions, it follows that they often have very different characteristics. Most of these characteristics can be controlled by adaptively tuning the filter. However, even when an adaptive-tuning scheme is employed, the filtering structure of direct-conversion and other types of transceivers is not optimal because the receiver and transmitter filters are still implemented as separate components. Using separate filters substantially increases the size of the transceiver and thus undermines miniaturization.
In view of the foregoing considerations, it is apparent that there is a need for a system and method which may be implemented to more efficiently filter signals in the transmitter and receiver portions of a communications transceiver, and which achieves this improved performance with a greater degree of integration compared with conventional circuits of this type.